In high-energy nucleus–nucleus (AA) collisions, a hot and dense state of deconfined strongly-interacting matter, commonly denoted as the quark–gluon plasma, is formed [1]}.
This system undergoes hydrodynamic evolution [2]}, [3]}, [4]} and exhibits hadron yields indicating chemical equilibrium [5]}, [6]}.
At low to intermediate transverse momentum (\(p_{\mathrm {T}}\) ), up to about a few \(\mathrm {GeV}/c\) , charged-particle production is influenced by the collective expansion (flow) of the system, reflected in the shapes of single-particle transverse-momentum spectra [7]}, [8]} and multi-particle correlations [2]}.
It came as a major surprise that even small collision systems such as proton–proton (pp) and proton–nucleus (pA) collisions at LHC energies exhibit features that can be attributed to collective expansion [10]}, [11]}, [12]}, [13]}, [14]}, [15]}, [16]}, [17]}, [18]}.
